The aggregation behavior of carboxymethyl chitosan (CM-chitosan) with various degrees of deacetylation (DD) and substitution (DS) was characterized with viscometry, gel permeation chromatography (GPC), and GPC coupled with laser light scattering (GPC-LLS). The results indicate that CM-chitosan has a strong tendency to form aggregates in aqueous solution and the aggregation behavior depends on DD and DS values. The apparent aggregation number (N-ap), the gyration radius (R-g), and the weight fraction of the aggregates (F-a) reached maximum at a DD value of 50%, then decreased, with the DD value deviating from 50%. A higher DS value helped to form aggregates; when the DS value increased from 0.65 to above 1.0, N-ap and R-g increased sharply. The dependence of the refractive index increment (dn/dc) on the DD and DS values was related to variation of the charge density and the hydrophobic interaction along the molecular chains. The conformations of CM-chitosan aggregates were studied by the LLS method. The aggregates showed a spherical shape, and the chain stiffness increased with introduction of the acetyl groups. The DS value had no clear influence on the chain conformation that was observed. (C) 2004 Wiley Periodicals, Inc.